1. Field of the Invention
The invention relates to a technology for detecting current in a battery mounted on an electric vehicle or the like. More particularly, the invention relates to a technology for detecting charging/discharging current of a battery, which is used, for example, for calculating the remaining capacity of the battery.
2. Description of the Related Art
The remaining capacity of a secondary battery mounted on an electric vehicle or the like needs to be detected with high accuracy. For example, the remaining capacity is detected by accumulating charging/discharging current of the secondary battery. Therefore, in order to detect the remaining capacity of the secondary battery with high accuracy, it is necessary to detect charging/discharging current of the secondary battery with high accuracy. When current sensor for detecting charging/discharging current of the secondary battery is provided with an offset value, a detection error is caused by the amount corresponding to the offset value, and detection errors are accumulated. As a result, a large error is caused in the remaining capacity.
The offset value for the current sensor mounted on the electric vehicle or the like fluctuates with time, which cannot be ignored. Due to the time fluctuation, an error, which cannot be ignored, is caused in the remaining capacity. As a result, there is a possibility that problems such as overcharging and overdischarging occur, and performance of the secondary battery deteriorates. As such a current sensor, a semiconducting magnetometric sensor capable of detecting current with high accuracy is generally used. An external magnetic field such as the earth magnetism, magnetic flux leaking from a motor, a relay, a transformer, or coil near the sensor acts on the semiconducting magnetometric sensor provided near a conductor subject to detection. The offset value is generated due to the magnetic flux which acts on the semiconducting magnetometric sensor. The offset value fluctuates slightly due to movement of the vehicle. When charging/discharging current of the secondary battery are accumulated in order to detect the remaining capacity of the secondary battery, although the charging current and the discharging current cancel each other, the offset values are accumulated with time. Accordingly, the offset value gradually increases, and a large error is caused in the remaining capacity.
Assuming the offset value is constant and does not fluctuate, it is necessary only to measure and store the offset value, and to subtract the offset value from the measured value of the charging/discharging current. With the electric vehicle, however, a magnetism source inside the vehicle and a magnetic state outside the vehicle fluctuate spatially and with time, which causes fluctuation in the offset value with time. Therefore, it is impossible to completely cancel the offset value using the stored offset value. Since the electric vehicle itself moves, it is difficult to avoid fluctuation in the offset value due to a change in the environment surrounding the current sensor. In order to solve the above-mentioned problem, Japanese Patent Laid-Open Publication No. 11-150873 discloses a control device for an electric vehicle, which can detect the capacity of a secondary battery for an electric vehicle with high accuracy.
The control device for an electric vehicle disclosed in Japanese Patent Laid-Open Publication No. 11-150873 includes a running motor control circuit which is supplied with electric power from a secondary battery and in which an output from a running motor is controlled; a current detection circuit in which charging/discharging current of the secondary battery is detected; a remaining capacity calculation circuit in which the remaining capacity of the secondary battery is determined based on the detected discharging current; a secondary battery charging/discharging control circuit in which charging/discharging of the secondary battery is controlled based on the determined remaining capacity; a non-charging/discharging state detection circuit in which a predetermined non-charging/discharging state of the secondary battery is detected without using the current detection circuit; and an offset value detection storage circuit in which an output value of the current detection circuit is detected every time the non-charging/discharging state is detected and the detected output value is stored as an offset value for correcting charging/discharging current.
According to the control device, the predetermined non-charging/discharging state of the secondary battery is detected by a circuit other than the current detection circuit, and the offset value for a current sensor which detects discharging current of the secondary battery mounted on the electric vehicle is detected every time the predetermined non-charging/discharging state is detected. Accordingly, even when the offset value fluctuates with time due to spatial movement of the electric vehicle, a change of the current sensor with time, and the like. it is possible to detect charging current and discharging current of the secondary battery mounted on the electric vehicle with high accuracy. Therefore, it is possible to solve problems that the remaining capacity of the secondary battery is inaccurately estimated by accumulating errors in the offset value, which results in occurrence of overcharging or overdischarging of the secondary battery, or hindrance to effective use of the secondary battery.
However, in the control device disclosed in Japanese Patent Laid-Open Publication No. 11-150873, the error in the offset value, which is caused due to a hysteresis characteristic of a core, is not taken into consideration. The hysteresis characteristic differs from core to core used in the current sensor. When the offset value is calculated without taking the hysteresis characteristic into consideration, the value includes an error due to the hysteresis characteristic. Accordingly, an accurate offset value cannot be calculated.